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					2D(3D?) cluster counting with
    GEMs and small pads:
      the digital TPC?


how to measure dE/dx without
     measuring charges
               Michael Hauschild (CERN), LCWS02, 27-Aug-2002, page 1
 Ideal dE/dx measurement
Count number of clusters along track
 cluster density should be proportional to dE/dx

Obvious problem:
 cluster density is high (20 - 30 clusters/cm in Ar mixtures for
 m.i.p.) = 1 cluster per 300 - 500 ìm
 need device with high granularity to resolve them in space

Other problem:
 Clusters sometimes have more than one electron:
    1 el. 82.4 %
                       (TESLA-TPC, Ar/CH4/CO2, 93/5/2, calculation by HEED)
    2 el.   6.9 %
    3 el    2.0 %
                                 how to avoid counting individual
   10 el.   0.64 %
  100 el.   0.0014 %
                                 electrons of multi-electron clusters?

                                             Michael Hauschild (CERN), LCWS02, 27-Aug-2002, page 2
  Classic dE/dx measurement
Widely used (because counting is difficult):
  measure charge over some track length (sampling length)
  "average" charge of many samples = dE/dx

Charge measurement requires:
  electronics with good charge resolution, e.g. 8 bit or more
  stable gain!!!
    physics needs ÄG/G < 1/10ó(dE/dx)/dE/dx (better < 1/20)
       < 0.5% (better < 0.2%) overall gain stability

  some algorithm to remove unwanted multi-electron clusters
  (delta electrons)
    commonly used: truncated mean, remove a fixed fraction of highest charge
    measurements, typically 20-30%, robust

                                     Michael Hauschild (CERN), LCWS02, 27-Aug-2002, page 3
               dE/dx with GEMs
GEMs (Gas Electron Multiplier) + pads (typical size 5-10 mm)
are suggested as possible TPC-readout device
Nice detector for tracking, what about dE/dx?
Problem: GEMs show gas gain variations
  local variations over the surface (static)
    could make calibrations more complicated, not a problem in principle
  time dependent (dynamic) variations due to charge-up effects
    difficult to control and to calibrate, depend on background, might vary
    within a bunch train

                                       Michael Hauschild (CERN), LCWS02, 27-Aug-2002, page 4
                   GEM gain variations
local gain variations:                     dynamic gain variations:

                                                              COMPASS GEMs

M. Hamann et al.(DESY/Univ. Hamburg)         C. Altunbas et al., CERN-EP 2002-008

➔   10% local gain variations          ➔   20-30% dynamic gain variations

                                           Michael Hauschild (CERN), LCWS02, 27-Aug-2002, page 5
              Cluster Counting
Direct cluster counting avoids any problems with
gas gain instabilities
In theory  ultimate way to get dE/dx
 30 clusters/cm * 120 cm track length = 3600 clusters = 1.7%
 dE/dx resolution (TESLA-TDR: 4-4.5%)

Not a brand new idea:
 previous attempts tried to resolve clusters in time:
   slow gas / drift velocity (e.g. CO2) + good time and multi-hit resolution,
   worked in lab + prototype detectors, never used in real big detectors for

Now (that's new):
 micro-pattern devices + small pads = high granularity could
 make it possible to resolve them in space (2D), if time could be
 added  even 3D(?)
                                      Michael Hauschild (CERN), LCWS02, 27-Aug-2002, page 6
                Could it work?
Simulation study made:
  generate clusters/electrons (also long range delta electrons
  using HEED (by I. Smirnov), take gas parameters (diffusion
  etc.) from MAGBOLTZ (by S. Biagi)
  track electrons through TPC volume, squeeze them through
  GEM holes, apply gas gain (use Polya distribution for
  track all electrons created in gas amplification to a pad plane
  (including diffusion, ódiffusion = 135 ìm over 2 mm)
  collect electrons on pads, allow 5% losses in gaps, add noise
  (200 el. R.M.S. per pad, optimistic?)
  apply threshold (1500 el.) and simply count number of pads
  above threshold = clusters(?)
very CPU time consuming, need 50 Mill. electrons per full TPC
                                 Michael Hauschild (CERN), LCWS02, 27-Aug-2002, page 7
    Generated Electrons

                                    HEED calculation

                                     some delta-

             TPC frame
             (sideview)            GEM plane

                    Michael Hauschild (CERN), LCWS02, 27-Aug-2002, page 8
Generated Electrons (close view)

           65 keV

    no diffusion                                          with diffusion
     (ódiff,trans. = 1.1 mm, ódiff,long. = 4.4 mm for 250 cm drift and 4 T)

                                         Michael Hauschild (CERN), LCWS02, 27-Aug-2002, page 9
    Pad view (xy plane)


single electrons                pads (500 x 500 ìm2) above
 at GEM plane                      threshold (1500 e-)

                        Michael Hauschild (CERN), LCWS02, 27-Aug-2002, page 10
Counting with small pads seems to work
Some questions to answer:
 optimal pad size?
   large pads: clusters can't be resolved
   small pads: (too) many channels (cost!), bad signal/noise ratio
   need low threshold to count a single electron after gas amplification on a
   single small pad
   at large drift length (up to 250 cm at TESLA-TPC), multi-electron clusters
   are spread by diffusion, individual electrons appear and are counted
   again, not clusters

                                      Michael Hauschild (CERN), LCWS02, 27-Aug-2002, page 11
                  Counts vs Pad Size

m.i.p., 0.6 GeV pions             high E tracks, 1000 GeV pions
                                          (Fermi plateau)

                           Michael Hauschild (CERN), LCWS02, 27-Aug-2002, page 12
          Separation Power

simple pad counting,       classic dE/dx by charge
2.1 ó separation           measurement + truncated
                           mean, 2.1 ó separation
                       Michael Hauschild (CERN), LCWS02, 27-Aug-2002, page 13
                   Conclusions I
Separation power with cluster counting as good as
classic dE/dx by charge + truncated mean...
...but not better! (Factor 2 improvement expected)
    Need to match:        average distance
                          between clusters
                            (375 ìm for m.i.p.)

        diffusion spread                         pad size
      (1.1 mm for 250 cm drift)               (noise  cost)

➔   Doesn't fit together for TESLA-TPC, too much diffusion!
➔   Electrons from multi-electron clusters dissolve and are
    individually counted (instead of clusters)
                                      Michael Hauschild (CERN), LCWS02, 27-Aug-2002, page 14
                   Conclusions II
Cluster counting better than classic dE/dx by
charge needs:
 gas with low ionisation
   large distance between clusters
   low probability for multi-electron clusters
 low diffusion
   either low diffusion gas or
   short drift length
 pad sizes

Helium(Neon?) mixtures could be a possible
candidate for successful cluster counting

                                       Michael Hauschild (CERN), LCWS02, 27-Aug-2002, page 15

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